Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers, and even electric cars, development of a rechargeable secondary battery, in particular, a lithium secondary battery is the focus of particular interest.
However, a porous separator of a secondary battery shows severe thermal shrinkage behaviors at a temperature of 100° C. or higher in view of its material properties and features of a manufacturing process including stretching, causing a short circuit between a cathode and an anode. To address a safety issue of a battery, for example, Korean Patent Publication No. 2007-0083975 (Hitachi) and Korean Patent Publication No. 2007-0019958 (Evonik) disclose a separator in which a porous coating layer formed from a mixture of insulating filler particles and a binder polymer is provided on a porous substrate and a material having a shut-down function is added to the porous coating layer.
However, when a secondary battery is, for example, overcharged, a transition ion (metal ion) in excess is released from a cathode and inserted into an anode, so that the transition ion having high reactivity is deposited on the anode surface and the cathode becomes thermally unstable. Moreover, the battery still has unresolved safety concerns such as overheat, fire, or explosion due to drastic heat generation caused by a decomposition reaction of an organic solvent used as an electrolyte solution. Accordingly, there is a need for a new coating layer having a function of removing excess metal ions that become problematic with safety.
Also, there is still a demand for a separator for a secondary battery in which a pore structure is well maintained in each of a porous coating layer having inorganic particles and a separator in contact with the porous coating layer and a transition ion can be favorably moved between the porous coating layer and the separator by the pore structure.